Dehydrocyclization as a unit process has held a prominent place in the petroleum refining industry for some time. Quite broadly the process encompasses the conversion of C.sub.2 -C.sub.12 olefins and paraffins to aromatics and naphthenes, but the process can be divided into two segments on both chemical mechanistic grounds and refinery operational considerations. In one branch light paraffins and/or olefins in the C.sub.2 -C.sub.5 range are converted to naphthenes and aromatics, necessarily of higher carbon number, by a sequence of reactions including dehydrogenation, dimerization and oligomerization of olefins, and cyclization. Typically the process is referred to as dehydrocyclodimerization, which clearly indicates the various component processes occurring. See S. M. Csicsery, Ind. Eng. Chem. Proc. Des. Dev., 18, 191 (1979). In another branch olefins and/or paraffins of 6 or more carbon numbers are converted to naphthenes and aromatics of the same carbon number or less. It is this last branch which is of greatest interest to us here.
The dehydrocyclization of paraffins to naphthenes and aromatics is a difficult reaction, limited by an equilibrium which shifts somewhat toward cyclization as the molecular weight (i.e., chain length) of the paraffin increases. Dehydrocyclization is favored by low pressures and high temperatures and may be catalyzed by, e.g., dual functional catalysts having both metal and acid functions. See J. A. Weiszmann, "Handbook of Petroleum Refining Processes", R. A. Meyers, ed., McGraw-Hill Book Company, 1986, page 3-8. Two types of catalysts have been identified for dehydrocyclization of paraffins; dual functional catalysts as described above and monofunctional catalysts possessing no significant acidity. The latter class of catalysts generally contains platinum on a neutral or a basing support, and includes such materials as platinum on a zeolite where all of the zeolitic activity has been removed by exchange with alkali or alkaline earth metal cations such as potassium sodium, barium, and so forth. More recently, N. Y. Chen and T. Y. Yan, Ind. Eng. Chem. Proc. Des. Dev., 25, 151 (1986) have distinguished between two mechanistic routes for the aromatization of light hydrocarbons based on studies performed with n-hexane. One reaction path is a more or less direct road from the paraffin to aromatics, largely benzene, as effected over such catalysts as Pt/Al.sub.2 O.sub.3, Cr.sub.2 O.sub.3 /Al.sub.2 O.sub.3, CoMo/Al.sub.2 O.sub.3 and Te/NaY. The other mechanistic route involves a pathway toward cracked intermediates which subsequently react to afford aromatics. The latter is said to be characteristic of HZSM-5 as catalyst and can be recognized by a spectrum of C.sub.6 -C.sub.10 aromatics in the product stream from n-hexane.
While exploring dehydrocyclization under a range of conditions we pondered the possibility of using amines as a feedstock instead of the paraffins normally used. Various reactions can be contemplated, and without a hint from the prior art as to appropriate catalysts or probable reaction products we performed a general survey. In this application we report on our results on the dehydrocyclization of propylamines. In particular, we have found that under appropriate reaction conditions one can prepare azacycloheptanes from secondary and tertiary propylamines. Since the seven-membered nitrogen heterocycle is otherwise difficult to prepare, especially in large commercial quantities, our new synthesis provides a valuable entry into the family of azacycloheptanes.